Filters using such multi-mode surface acoustic wave devices are disclosed in Japanese Unexamined Patent Publication Nos. 2001-257556, 2000-224003, 11-317643, 10-51259, 8-191229, 8-125489, and 7-135444, for example.
Each of the multi-mode surface acoustic wave filters disclosed in those publications includes a substrate, a pair of reflection electrodes that are formed on the substrate, and drive electrodes that are interposed between the reflection electrodes. So as to improve the characteristics of those multi-mode surface acoustic wave filters, various techniques have been suggested. In the above publications, spurious signals that appear on the low frequency side and the high frequency side of the vicinities of the pass band are suppressed by improving the reflectors that form stop bands.
For example, Japanese Unexamined Patent Publication No. 2001-257556 discloses a structure in which three electrodes are provided for each wavelength of excited surface wave in each reflector so as to suppress spurious signals due to high-order longitudinal mode on the low frequency side of the pass band. Japanese Unexamined Patent Publication No. 2000-224003 discloses a structure in which the electrode pitches of one of the reflectors differ from the electrode pitches of the other one of the reflectors so as to suppress spurious signals that appear on the low frequency side of the pass band. Japanese Unexamined Patent Publication No. 11-317643 discloses a structure in which the electrode pitch of one of the reflectors differs from the electrode pitch of the other one of the reflectors so as to suppress spurious signals that appear on the low frequency side of the pass band. Further, Japanese Unexamined Patent Publication Nos. 10-51259, 8-191229, 8-125489, and 7-135444 disclose improvements based on the distance between a comb-like electrode and each reflector.
However, those publications do not disclose a technique for improving the suppression on spurious signals and the shape factor (the sharpness of the cut-off region of the pass band).
Therefore, the object of the present invention is to provide a surface acoustic wave device that simultaneously exhibits greater suppression on spurious signals and an improved shape factor (the sharpness of the cut-off region of the pass band) by a different method from any of the above conventional methods.